Electroluminescent (EL) devices, known as self-luminous displays, provide a wider viewing angle, higher contrast, and shorter response time. EL devices can be classified depending on the material used for the electroluminescent layer as inorganic or organic EL devices. As compared with inorganic EL devices, organic EL devices provide higher luminance, lower driving voltage, shorter response time, and the ability to display a wider range of colors.
A typical organic EL device includes an anode on the top surface of a substrate. A hole transporting layer, an electroluminescent layer, an electron transporting layer, and a cathode are formed sequentially on the anode. The hole transporting layer, the electroluminescent layer, and the electron transporting layer are thin films made of organic compounds.
Organic EL devices with the above-described structure operate according to the following principles. When a voltage is applied across the anode and the cathode, holes injected from the anode migrate via the hole transporting layer into the electroluminescent layer. Electrons injected from the cathode migrate via the electron transporting layer into the electroluminescent layer and combine with the holes therein to generate excitons. When the excitons transit from an excited state to a base state, molecules in the electroluminescent layer emit light to form visible images. Light emission occurring as excitons transiting from a singlet state (S1) to a base (S0) state is referred to as “fluorescence”, and light emission occurring as excitons transiting from a triplet (T1) state to a base state is referred to as “phosphorescence”. In fluorescence, only 25% of the singlet state excitons (75% of triplet state excitons) are used, thereby limiting emission efficiency. In contrast, in phosphorescence 75% of triplet state excitons and 25% of singlet state excitons are used so that 100% internal quantum efficiency can be theoretically achieved.
A high-efficiency, green and red organic EL device has been developed using Ir(ppy)3, which is a phosphorescent colorant having a heavy atom such as Ir or Pt with strong spin-orbit bond and PtOEP as dopants to enable effective light emission in a triplet (phosphorescent) state. In the organic EL device CBP (4,4′-N,N′-dicarbazole-biphenyl) is used as a host. This organic EL device, however, has a short lifespan of 150 hours because the CBP has a low glass transition temperature of 110° C. and is liable to be crystallized, making it unsuitable for commercial use.